As a shock absorber mounted on a vehicle such as an automobile, a shock absorber which changes a damping force by changing apparent viscosity of a magnetorheological fluid by causing a magnetic field to act on a flow passage through which the magnetorheological fluid passes. JP2008-175364A discloses a magnetorheological fluid shock absorber in which, when a piston assy provided with a piston core around which a coil is wound on an outer periphery and a piston ring arranged on the outer periphery of the piston core slides through a cylinder, a magnetorheological fluid passes through a flow passage formed between the piston core and the piston ring.